Author Topic: clipping problem  (Read 12428 times)

red_92

clipping problem
« on: November 29, 2014, 05:12:55 PM »
hello guys,over the last days I've been thinking about building/designing a new distortion pedal using more transistor boosting stages and a diode clipping stage
So,today I made a simple booster wits a MPSA18 ...it worked ok,so I added some diodes (one green LED and a germanium diode,symmetrical ) between the output and the ground - I get a distorted sound,but after rolling the output pot (I mounted a pot between ground and the transistor's collector) over 40% the sound dies

This means that after the signal exceeds a certain level something goes wrong,why, any ideas?

duck_arse

Re: clipping problem
« Reply #1 on: November 30, 2014, 09:56:43 AM »
can you draw up a circuit diagram for what you've built, so we can see what it is you're doing?
"I think we're done here."

red_92

Re: clipping problem
« Reply #2 on: November 30, 2014, 03:25:38 PM »


something like this,it's a very basic booster with 2 diodes,i'm still learning and testing,so it's not very refined  ??? ???

the whole idea is to cascade 2 or more boosting stages so i'll get a strong signal before entering the clipping stage,hoping to get a heavy distortion

bluebunny

Re: clipping problem
« Reply #3 on: December 01, 2014, 04:01:06 AM »
. . .but after rolling the output pot over 40% the sound dies

What happens if you short the collector to the wiper of the pot?
  • SUPPORTER
Ohm's Law - much like Coles Law, but with less cabbage...

red_92

Re: clipping problem
« Reply #4 on: December 01, 2014, 05:56:02 AM »
I get no sound at  all by shorting the collector with the wiper
Today I took out the pot from the circuit and connected the diodes directly between the collector and ground and again,no sound.
The schematic works well as a booster (without diodes) but when adding the clipping parts,it acts strange

mpsa18 got a maximum hfe 1500,while a 2n3904 only 300 - i know the circuit works well with a 2n3904 because i've built it some years ago-so, is the problem related somehow to his?

Hatredman

Re: clipping problem
« Reply #5 on: December 01, 2014, 06:46:38 AM »
Did you test the diodes to see if one of them is shorted?
Kirk Hammet invented the Burst Box.

slacker

Re:
« Reply #6 on: December 01, 2014, 07:01:38 AM »
Your schematic doesn't show input or output capacitors, have you fitted these? Without an output cap the diodes will pull the collector voltage down to about 0.6 Volts effectively shorting out the transistor so it won't work.

Hatredman

Re: clipping problem
« Reply #7 on: December 01, 2014, 07:21:18 AM »
Goot catch! The capacitor between the collector and the por is a must.
Kirk Hammet invented the Burst Box.

duck_arse

Re: clipping problem
« Reply #8 on: December 01, 2014, 08:31:12 AM »
well, I'll go out on a limb and say that with the diodes shorted to the collector, one will be forward biased, and the collector voltage will therefore be 0V6. so, the base wants to be ~0V6 above the emitter, and the emitter has a resistor in series, so it will have a voltage drop of ??V across that. therefore, there can't be enuff volts to bias the base, so nothing happens, exept for the drain on the battery via the 3k3 and fwd biased diode.

circuit analysis, fun innit? did I get any of that right?

you need at the very least a resistor in series with the collector and VR. really, it should be a dc blocking cap before the pot (and between the input and R2//transistor base), so they only work at the AC conditions, and a series resistor before the diodes, to maybe limit their current. (most of this came up in a recent thread on tripling the lpb1.)
"I think we're done here."

red_92

Re: clipping problem
« Reply #9 on: December 02, 2014, 08:14:55 AM »
thank you guys,I added the input and output cap and it works well now-with only one transistor, the sound it's more crunchy so the next step will be to add one or two boosting stages before the clipping diodes
the clipping part it's going to have two sets of different diodes,with the possibility of choosing the ones that you want via a switch.and another switch which allows you to select between symmetrical and asymmetrical clipping
I'll keep you updated

Quackzed

Re: clipping problem
« Reply #10 on: February 20, 2015, 11:28:18 AM »
try a 10k ish resistor between the vol wiper and the diodes...   wiper---10k---diodes to ground---out
nothing says forever like a solid block of liquid nails!!!

Jdansti

Re: clipping problem
« Reply #11 on: February 21, 2015, 04:04:09 AM »
Shouldn't the diodes be before the volume pot?
R.G. Keene: EXPECT there to be errors, and defeat them...

bool

Re: clipping problem
« Reply #12 on: February 21, 2015, 04:22:03 AM »
There's no "must be" to where diodes should go.

A mere elegent solution would be to put diodes before the pot, but then you almost always have to deal with a "gain" control, so with yet another pot.

Putting diodes after a "vol" pot is a simplistic way to stack "two controls in one", or a (God forbid) faux-two stage distortion (if you count a transistor as a first stage "dist") and a diode clipper as the second "dist".

IOW you have a "two-in-one" knob.